Cellulose is a renewable natural polymer that can be converted to many chemical derivatives. The derivatization takes place mostly by chemical reactions of the hydroxyl groups in the β-D-glucopyranose units of the polymer. By chemical derivatization the properties of the cellulose can be altered in comparison to the original chemical form while retaining the polymeric structure. Reaction selectivity is important so that a derivative of desired chemical structure could be obtained.
Heterocyclic nitroxyl compounds are known as catalysts that participate in the selective oxidation of C-6 hydroxyl groups of cellulose molecules to aldehydes and carboxylic adds, the corresponding oxoammonium salt being known as the active direct oxidant in the reaction series. One of these chemical oxidation catalysts known for a long time is “TEMPO”, i.e. 2,2,6,6-tetramethylpiperidinyl-1-oxy free radical. Thus, the oxidized forms of the nitroxyl radicals, N-oxoammoniumions, act as direct oxidants in the oxidation of the target cellulose molecule, whereas a main oxidant is used to bring oxygen to the reaction series and convert the nitroxyl compound back to the oxidized form.
It is known to oxidize primary alcohols to aldehydes and carboxylic acids through “TEMPO” by using sodium hypochlorite as the main oxidant (for example Anelli, P. L.; Biffi, C.; Montanan, F.; Quici, S.; J. Org. Chem. 1987, 52, 2559). To improve the yield in the oxidation of the alcohols to carboxylic acids, a mixture of sodium hypochlorite and sodium chlorate was also used (Zhao, M. M.; Li, J.; Mano, E.; Song, Z. J.; Tschaen, D. M.; Org. Synth. 2005, 81, 195).
It is also known procedure to catalytically oxidize cellulose in native cellulose fibers through “TEMPO” by using sodium hypochlorite as main oxidant (oxygen source) and sodium bromide as activator (Saito, T. et al.; Cellulose Nanofibers Prepared by TEMPO-Mediated Oxidation of Native Cellulose, Biomacromolecules 2007, 8, 2485-2491). The primary hydroxyl groups (C6-hydroxyl groups) of the cellulosic β-D-glucopyranose units are selectively oxidized to carboxylic groups. Some aldehyde groups are also formed from the primary hydroxyl groups. When the fibers of oxidized cellulose so obtained are disintegrated in water, they give stable transparent dispersion of individualized cellulose fibrils of 3-5 nm in width, that is, so-called nanofibrillar cellulose.
Selectivity of the oxidation is important so that chemicals used are not consumed to unwanted side reactions. Selectivity can be defined as ratio of carboxylic groups formed to the main oxidant consumed.
The reaction rate of the catalytic oxidation is primarily dependent on the concentration of the heterocyclic nitroxyl catalyst, the concentration of the main oxidant, pH, reaction temperature, and the pulp consistency (concentration of the pulp in the reaction medium). The properties of the pulp are also significant, for example the availability of the unoxidized cellulose to reactants is an important factor. Amorphous cellulose is typically easier to oxidize than crystalline cellulose, as amorphous cellulose is more accessible to water.
Although it is possible to oxidize the cellulose to the desired oxidation level, as expressed for example by mmol COOH/g pulp, the problems may arise in the selectivity of the oxidation (COOH groups formed/consumed main oxidant such as NaClO). Low oxidation selectivity will cause increased consumption of the main oxidant. Low oxidation selectivity also correlates with low oxidation rate due to the decomposition of NaClO and the direct reaction between NaClO and pulp during the oxidation process, which tends to split the cellulose and lower the DP. Most selective oxidations of cellulose can be performed with reactive pulps. Never-dried pulps are good examples of reactive pulps. The higher reactivity of never-dried pulps compared with dry pulps is due to the cellulose microfibril aggregation that takes place as water is removed from the cellulosic material, which reduces the amount of accessible hydroxyl groups of cellulose in the pulp.